Are tech companies turning people away due to their race? Or are there simply not enough black programmers? If the latter, then diversity for the sake of it is going to be... strange.
The National Bureau of Economic Research did an experiment where they sent in the same résumé for the same job under "white-sounding" and "black-sounding names": the "white" names got about 50% more call-backs than the "black" names. [1]
Granted, this wasn't aimed at tech companies.
[1] http://www.nber.org/digest/sep03/w9873.html